The present invention relates to oscillators, and more particularly to an operational amplifier oscillator for implementation as an integrated circuit.
Most oscillator circuits use discreet devices, such as individual transistors. Such oscillators generally have excellent phase noise characteristics. However more compact circuits with fewer elements are needed.
Another reoccurring problem, especially in the design of crystal oscillators, is to control the amount of energy dissipated in the resonator that determines the operating frequency. Crystal circuits must strictly limit the amount of energy dissipated in the crystal, typically about 1 mW. The same is true in LC oscillators using varactor diodes. The RF voltage across the varactor must be carefully limited to prevent forward biasing the diode.
A further problem occurs in voltage tuned crystal oscillators (VXCO) where a considerable amount of gain is sometimes required for operation across the tuning range to overcome the losses caused by component and board losses. Any loss creates an equivalent resistance that appears in series with the quartz crystal. Multiple transistors are required to achieve sufficient gain, causing design complication and consuming large circuit board areas devoted to the VXCO function.
Most integrated circuits used to create oscillators take the form of the typical microprocessor clock circuitxe2x80x94the I/O terminals of logic gates are provided to be configured as oscillators. These resonator circuits typically use a shunt capacitor and a series quartz crystal followed by another shunt capacitor.
What is desired is an oscillator implementable in an integrated circuit having a simple topology that allows independently setting the resonator power and gives low to very low phase noise.
Accordingly the present invention provides an operational amplifier oscillator that uses an operational amplifier to provide a low impedance source for a resonator while providing an output voltage signal at the desired frequency. The operational amplifier has positive and negative feedback paths, with the negative feedback path having a first path for driving the input impedance at the negative input to a small value when the output voltage signal is near a zero crossing and having a second path for driving the input impedance even smaller when the output voltage signal swings away from the zero crossing. A filter between the output and the positive feedback path is used to minimize phase noise by blocking low frequency noise in the output of the amplifier from the positive input of the amplifier.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in light of the appended claims and attached drawing.